1. Field of the Invention
The present invention generally relates to the field of ultraviolet microscopes. More particularly, the present invention relates to a microscope for observing a specimen in a wavelength range from a visible range to an ultraviolet range, a microscope for superposing an ultraviolet picture of a specimen on a visible color picture thereof and displaying a superposed picture, and a microscope for observing only an ultraviolet image of a specimen.
2. Description of the Related Art
In a lithographic process for semiconductor devices, an exposed state of a semiconductor device is observed in a microscopic examination. In a test using an optical microscope with a visible light source, a structure having a size of 0.8 .mu.m or more can be observed, and interference colors of a resist film can be tested.
A scanning electron microscope (SEM) and an ultraviolet microscope are used to observe microstructures.
Along with the recent developments of micropatterning techniques, the structures of semiconductor devices tend to be further miniaturized. For example, in a 16-Mbyte dynamic type random-access memory (RAM), a line width is about 0.5 .mu.m. Since a resolving power of an optical microscope is insufficiently low for micropatterned devices, measurement of line widths and detection of defects cannot be performed. On the other hand, although an SEM and an ultraviolet microscope have sufficiently high resolving powers, pictures to be displayed are only monochrome pictures. Color information which is one of the most important test factors cannot be obtained. In addition, a vacuum atmosphere is required in the SEM during observation, thus complicating the operations.
The limit of the resolving power of the optical microscope depends on the wavelength of light from a light source used together with the optical microscope. A wavelength .lambda. of light used in an optical microscope is generally fixed to an average wavelength (i.e., 550 nm) of visible light as a peak of spectral luminous efficiency of a human eye. In this manner, when the wavelength .lambda. is determined to be the fixed value, a numerical aperture NA of an objective lens system must be increased to decrease a resolving power .epsilon. of the optical microscope, as can be apparent from the following equation: EQU .epsilon.=K.multidot..lambda./NA (i)
(where K is a proportional constant of 1 or less, .lambda. is the wavelength of light, NA is the numerical aperture of the objective lens system).
In a visible light microscope having a large magnification, an immersion objective lens system is particularly used to increase the numerical aperture NA of the objective lens system. However, it is cumbersome to immerse a specimen to be examined and the objective lens system in a liquid. In particular, in an oil immersion objective lens system, the object is contaminated. It is difficult to apply the immersion objective lens system to an inverted microscope due to the limitation of the position of the object.
On the other hand, as is apparent from equation (i), use of an ultraviolet light source having a short wavelength is more advantageous than use of a visible light source to obtain a higher resolving power. For example, since the wavelength of the spectral line of a mercury lamp is 275 nm, a resolving power is doubled with respect to a visible light source (average wavelength: 550 nm).
Quartz and fluorite are known as optical materials for efficiently transmitting ultraviolet rays therethrough.
For example, in an ultraviolet microscope disclosed in Published Unexamined Japanese Patent Application No. 64-62609, a mercury lamp is used as a light source. All lens systems such as an objective lens system and an eyepiece system in an optical path of an ultraviolet ray from the mercury lamp are made of quartz.
Quartz and fluorite, however, cannot correct chromatic aberrations of light ranging from the ultraviolet range to the visible light range. For example, in a microscope disclosed in Published Unexamined Japanese Patent Application No. 61-189515, an excimer laser source as a substantially monochromatic light source is used to solve the problem of chromatic aberrations.
Illumination light used for observation is limited to a single wavelength regardless of types of light sources. This cannot cope with observations in a wide wavelength range. For example, observations ranging from the visible range to the ultraviolet range cannot be performed, and positioning of an object under visible light cannot be performed.
Since each of the microscopes disclosed in the above prior arts has only one observation optical path, simultaneous observation using a plurality of observation units cannot be performed.
An ultraviolet reflection microscope using a reflection objective lens is also known to those skilled in the art. This microscope can cope with a light source having a wide wavelength range because reflection does not depend on the wavelengths.
Since the reflection objective lens, however, is made of two reflecting surfaces each having a curvature, the curvature of field cannot be corrected, and a good image cannot be obtained in the peripheral area of the visual field.